Variation in the body growth parameters of the ghost crab Ocypode quadrata from morphodynamically distinct sandy beaches

Abstract

Investigations of patterns of distribution of sandy beach macrofauna show that dissipative beaches tend to concentrate higher richness and biomass at the intertidal zone, as well as better fitness and larger sizes of a species, than reflective beaches. For supralittoral crustaceans, these trends are inverted, as predicted by the habitat safety hypothesis (HSH). For ghost-crabs, however, higher population abundance has been observed on dissipative beaches, but their life-history traits and the response to different beach morphologies were not well defined. Here, variation in crab size and growth was evaluated over a one year period using indirect measures from three low-impact beaches on the northern coast of São Paulo State, southeastern Brazil. These beaches represent the three basic types of morphodynamics: dissipative (Costa beach), intermediate (Felix beach) and reflective (Puruba beach). At each site, five 2m wide transects were randomly selected each month, and assessed across the crabs' entire area of occurrence. The fit of the von Bertalanffy body growth model was compared among sites. For all three beaches, a main mode due to the settlement of juveniles (mm) was identified in August and September. In the following summer, according to growth estimates, the settlement group would be approximately the same size as the main mode (mm) that was recorded during the previous summer. Specifically, the growth parameters estimated were: Costa (dissipative) L∞= 46.36mm, K= 0.90year-1 and t0= -0.19; Félix (intermediate) L∞= 57.95mm, K= 0.87year-1 and t0= -0.12; Puruba (reflective) L∞= 53.01mm, K= 1.11year-1 and t0= -0.07. The body growth adjustments were similar between Puruba and Felix, but distinct from Costa. Growth performance indexes ranged from 3.29 (Costa) to 3.46 (Félix) and 3.49 (Puruba), indicating both the homogeneity of this parameter among distinct beaches and close agreement between the intermediate and reflective beaches. This study suggests that variations in the development of ghost crabs concur with those observed for other supralittoral, crustaceans and the habitat safety hypothesis, with higher individual fitness for reflective beaches.